Ad network Chitika has posted a running estimate of iPads in use. Chitika is calculating the number by counting the number of new unique iPad Safari browsers that have appeared on its ad network and then multiplying this number by the percent of the Internet that Chitika sees. Chitika also charts which states the iPads are coming from.

How accurate is the estimate? The methodology sounds reasonable, though the extrapolation to the broader Internet is obviously imprecise. Based on the sales numbers reported over the weekend, the resulting estimate sounds reasonable.